Symbolic representation probably based on Buddhavamsa 1.57-1.59 where the Buddha's chief disciples were compared to a special symbol, for example, Sariputta as the Koranda flower, Moggallana as the Blue Lotus, Mahakassapa as Pure Gold, etc.

Krishna is portrayed as blue. This may well be because of an early mistranslation of the word dark as blue. So the theory has it that Krishna was originally a southern Dravadian (hence seen as dark skinned) god adopted or assimilated into the pantheon of the paler Northern invaders/immigrants who eventually became dominant and codified their pantheon into more or less what we know in historic times. If so, the color blue may be associated with or signify holiness of a southern origin. On the other hand, they may just have had some extra blue paint on hand back in the day!

Probably due to a historical mistranslation of word "neela" which is used for both colors blue and black.
As in "neela warala" means "black hair (of a women)",
Compared to "neela peetha lohita" means "blue yellow red"

According to the Chinese versions of Kassapa Samyutta, Ven. Mahakassapa has the image of long beard-and-hair (SA1142 (in T99), ASA117 (in T100)). I've got this information from this article:

Choong, Mun-keat. 2017. 'A comparison of the Pali and Chinese versions of the Kassapa Samyutta, a collection of early Buddhist discourses on the Venerable Kasyapa', in Journal of the Royal Asiatic Society (Cambridge University Press), Vol. 27, Issue 2, pp. 295-311.